This invention relates to a time division multiple access system for use in a radio communication network and, in particular, in a multidirectional time-division multiplex radio communication network.
Such a time division multiple access system comprises a central station fixedly located at a predetermined terrestrial site and a plurality of substations geographically spaced on the earth from the central station, as will later be described with reference to one of some figures of the accompanying drawing.
In order to carry out communication between the central and each substation, a succession of down-link continuous signals is sent from the central station to be reproduced at each substation by selecting the down-link continuous signals allotted thereto while an up-link burst sequence is sent from each substation to the central station with the up-link bursts produced in a predetermined order under control of the central station to form at the central station a frame including a predetermined number of the up-link bursts.
Each up-link burst has been used for transmitting a main signal, such as a data signal, a speech signal, or the like, from each substation to the central station. In addition to the main signal, a subsidiary signal of an analog form, such as an order-wire signal, a supervisory signal, or the like, has been carried by each up-link burst.
With such a conventional system, a particular or common time slot has been assigned to each up-link frame and has been commonly used by the respective substations to transmit the subsidiary signals therefrom. The subsidiary signals are arranged during the common time slot in a digital form. An addition of the common time slot to the up-link frame gives rise to an inevitable increase of a clock repetition frequency used in this system and widens a radio frequency band. This means that unfavorable interference is induced into another system using an adjacent frequency band. In addition, the operation for controlling the subsidiary signals becomes intricate.
An application of an FM composite transmission method to the time division multiple access system may be contemplated to transmit the subsidiary signal in addition to the main signal. With such a transmission method, the subsidiary signal is subjected to frequency modulation under a low modification index with the main signal phase modulated.
Inasmuch as an up-link burst of each substation intermittently appears in each frame, the frequency modulated subsidiary signal is inevitably interrupted. Accordingly if, the frequency modulated subsidiary signal is to be continuously produced, it becomes discontinuous. Thus, the subsidiary signal can not favorably be transmitted with this method.